I would guess that it's stylistic. He's become known for his fucked up voice, so making it even more fucked up makes it more distinguishable. Kinda like those pop punk singers that you know could sound good if they tried but they make their voices all whiny and nasally to fit the style.

Tim sounds sleepy, like he took an Ambien before recording. He's doing that thing like the guy from the Strokes where you only put as much force into your voice as to get it past your lips, and no more.

Every Armstrong/Rancid song that gets posted, it's always the same idiots making the same endless comments about Tim's voice.

What's the point, really? Why do you feel the need to keep repeating yourselves? Have you got nothing better to do, nothing positive to contribute other than the same banal dimwitted jibe every single fucking time?